In recent years, extensive investigations have been made on the practical use of lactic acid polymers having excellent biodegradability as general-purpose polymers in a wide range of fields from the standpoints of alleviating environmental problems and others, and a large number of investigations and patent applications have been made concerning production processes therefor. However, the conventional polymers of lactic acid or lactide, i.e., poly(lactic acid) and copolymers of lactide and other monomer(s), have insufficient performance with respect to moldability and transparency. In addition, poly(lactic acid) has other problems, for example, that it degrades too quickly in applications other than special ones, so that it is unsuited for use as a general-purpose resin. There has hence been a desire for the development of a novel biodegradable polymer.
In International Publication No. 91/02015 are described a copolymer of an aromatic polyester, such as poly(ethylene terephthalate) or poly(butylene terephthalate), with either polyglycolide or poly(lactic acid) and processes for producing the copolymer.
The processes described are one based on the reaction of monomers in which lactide is reacted with butylene glycol and dimethyl terephthalate and one based on a reaction between polymers in which transesterification is conducted between two polymers, polyglycolide and poly(butylene terephthalate), at a temperature as high as 220.degree. C. However, the processes given in the Examples are limited to the method of transesterification between polymers.
In JP-A-4-504731 is described a process in which lactide is polymerized in the presence of poly(ethylene terephthalate) to produce a polymer blend of poly(lactic acid) and poly(ethylene terephthalate). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Further, techniques of reacting a lactone with a crystalline aromatic polyester are described in JP-B-48-4115 and JP-B-48-4116. (The term "JP-B" as used herein means an "examined Japanese patent publication".) In these processes, lactones, in particular .epsilon.-caprolactone and .gamma.-valerolactone, are reacted with a crystalline aromatic polyester.
However, the process described in JP-A-4-504731 is defective in that since the softening point of poly(ethylene terephthalate) is 220.degree. C. or above, which is higher than the decomposition temperature, 185.degree. C., of lactide, only a significantly discolored copolymer is obtained whose molecular weight is not high. The processes described in JP-B-48-4115 and JP-B-48-4116, which involve the reaction of a lactone, have problems, for example, that the copolymers obtained are not transparent, have excessive pliability, and are not preferred as a molding resin.
It is thus well known that a sufficiently high molecular weight cannot be attained with the hitherto reported processes for producing the desired polymer from monomers alone, that is, from a dicarboxylic acid component or an ester thereof, a diol component, and a cyclic ester such as lactide. With respect to the process for producing the desired polymer by reaction between polymers, i.e., an aromatic polyester, such as poly(ethylene terephthalate) or poly(butylene terephthalate), and poly(lactic acid), the process is impractical because the decomposition temperature of the latter polymer is far lower than the temperature at which the former polymer becomes flowable.
In addition, the lactic acid-based copolyester obtained is brittle and has poor transparency, which properties are attributable to the crystallinity and high melting temperature of the aromatic polyester and the poor compatibility thereof with other compounds. As a method for producing a copolymer of lactide and an aliphatic polyester, a process is described in JP-A-63-145661 which comprises polymerizing .epsilon.-caprolactone beforehand to obtain its homopolymer and block-copolymerizing lactide with the homopolymer.
The process in which lactide is block-copolymerized with poly(.epsilon.-caprolactone), however, is disadvantageous in that the copolymer obtained is cloudy and not transparent. The reasons why the copolymer is cloudy may be that in the copolymer the poly(.epsilon.-caprolactone) blocks are poorly compatible with the poly(lactic acid) blocks and the aliphatic polyester consisting of the poly(.epsilon.-caprolactone) blocks generally has high crystallizability and is pliable at room temperature despite its relatively high glass transition temperature as determined by differential thermal analysis.
To sum up the prior art techniques described above, the conventionally known poly(lactic acid), although excellent in degradability and transparency, not only has had problems that it degrades too quickly and has poor heat resistance, but also has had insufficient moldability for packaging applications such as films and sheets due to the rigidity attributable to the crystallization of lactide.
On the other hand, the conventionally known copolymers of lactic acid have been defective in that they have lower molecular weights than general-purpose non-degradable polymers and do not have transparency or sufficient heat resistance, although the copolymers are degradable. In addition, the copolymers with an aromatic polyester have had insufficient pliability for use as packaging materials such as films and sheets due to the rigidity of lactide.
If the residual lactide monomer is used as a plasticizer as a means for plasticization, there are problems, for example, that the lactide sublimes during the production process and the emitted lactide deposits on the production equipment to foul it, and that the elimination of the lactide as plasticizer from the polymer during storage or use results in disappearance of the plasticizing effect, smearing of the contents in the package, etc.
In the case of incorporating an ordinary plasticizer, it should be used in a large amount in order to attain desired plasticization and this inevitably results in the problem of plasticizer bleeding, with the problems during storage, i.e., disappearance of plasticizing effect during storage, smearing of the contents, etc., being left unsolved. Thus, a polymer usable as a packaging material having fully satisfactory properties has been unable to be obtained.